Proper lubrication is important to long gear life, especially in vehicle transmissions. Maintaining a proper oil level with the correct grade and type of oil is key to the extended life of gears and bearings in transmissions. Down times for changing of the oil and maintenance of the transmission contributes to truck down time, and is a major consideration for truck fleets as well as the individual owner operators. It has long been desired in the truck industry to have extended oil change intervals, which reduces down time due to maintenance.
In the past, transmission manufacturers of heavy duty trucks have recommended oil change intervals of 50,000 miles in heavy duty applications with petroleum based lubricants. With today's synthetic lubricants, these manufacturers have been able to extend those change intervals approximately five times, recommending change intervals of 250,000 miles after the original factory oil fill is drained and refilled with the synthetic lubricants at 5,000 miles or less. When the original factory fill is the new synthetic lubricant, no initial change is required. Several manufacturers are currently offering extended warranties for failure mode at 750,000 miles.
Until recently, petroleum based oils were used for lubrication in heavy duty truck transmissions even though they were susceptible to oxidation when operating the transmissions at oil sump temperatures above 230.degree. F. Many of the mineral gear oils break down above 230.degree. F. and oxidize, and thereby deposit carbonaceous coatings onto seals, bearings and gears that may cause premature failures. Consequently, regular oil changes were required in order to minimize oxidation and these deposits, to assure maximum component life and to maintain the warranties with the transmission manufacturers. The lower temperature limit and requirements for a transmission oil cooler restricted the success of the mineral gear oils to milder applications.
The new synthetic lubricants which are currently available can be operated at temperatures up to 250.degree. F., with intermittent operating temperatures up to 300.degree. F., without harming the transmission. If the average operating temperature is above about 250.degree. F., the transmission may require more frequent oil changes or external cooling. The following conditions, in any combination, can cause operating temperatures of over 250.degree. F. (1) operating consistently at slow speeds; (2) high ambient temperatures; (3) restricted air flow around the transmission; (4) having the exhaust system too close to the transmission; and (5) high horse power, over-drive operation.
Because many manufacturers design their transmissions so that the internal parts operate in a free flowing bath of oil which is circulated by the motion of gears and shafts, the oil droplet surface area is highly increased during operation, thereby rendering it particularly susceptible to oxidation. This oxidation helps to cause the accumulation of carbon deposits on the oil seal, which deteriorates the sealing material. It has long been established that there is a notable difference in seal life when using different lubricants. Once the seal material has deteriorated, the surfaces which mate to the seal may also be damaged or destroyed. These material failures cause the loss of lubricant over time, with an end result of low lubricant levels, and subsequent transmission failures. Fleet records have shown a failure rate of about 1 seal every 150,000 miles (on an average) during a normal maintenance period in a heavy duty truck transmission using mineral oil or petroleum based lubricants. The new synthetic lubricants have shown no evidence of leakage or recorded failure during the same maintenance in trucks, which indicate an extended seal life significantly beyond the capability of normal petroleum based lubricants.
Therefore, because petroleum based lubricants oxidize under the high operating temperatures to which transmissions are now being subjected, the oxidation problem we now experience causes deposits to form within the transmission, leaving a crusty carbon film throughout the transmission. These deposits adhere to bearings, gear hubs, tooth surfaces, sliding clutches, synchronizers and all other internal parts. High acid levels which develop during operation encourage this oxidation. Therefore, it is advisable to qualify various lubricants which will operate at higher temperatures with a lower rate of oxidation, lubricants which will experience lower acid levels, thereby encouraging less oxidation. Synthetic lubricants do not include the residual sulfur component that petroleum based lubricants generally do, and an approved synthetic lubricant will generally allow for extended drain intervals which gives the added benefit of reduced down time, and cleaner operating transmissions which give longer life.
In addition, the thermal aspects of today's truck design indicate that transmissions now operate at higher critical temperatures, requiring lubricants to perform under higher thermal conditions. Through research and testing, it has been determined that most petroleum based lubricants manufactured today have a lower thermal stability than those which are required during the high temperature operations needed by recent trucks. Due to the higher temperatures, and, as discussed hereinabove, the concomitant deposition of carbonaceous sludge and varnish and other accumulation of debris can become a factor in heat concentration, which possibly contributes to synchronizer failure, and ultimate transmission failure. Internal debris and deposits pose problems during transmission operation. Accumulation may lead to premature failure of the entire transmission unit. The extent of actual effect during operation may depend upon extremely careful maintenance, but the possible result is shortened transmission life due to excessive oxidation deposits.
In an attempt to overcome these thermal problems, new synthetic lubricants are being manufactured with thermally stable additive packages which are currently available from suppliers. Truck component manufacturers are encouraging oil suppliers to produce thermally stable additive packages for the synthetic lubricants which are more appropriate for today's truck designs which require higher temperature operating conditions. These suppliers of thermally stable additives blend the additives packages into synthetic base stock lubricants. The blended packages outperform other lubricants in the transmission because they are not only thermally stable, but the synthetic base stock does not generally thicken over time, allowing the longer drain intervals between oil changes.
Therefore, it has become apparent to transmission manufacturers that the generation of a procurement specification for qualifying lubricants has become imperative.
The old methods for testing lubricants included the ASTM designation D-130 standard test for detection of copper corrosion from petroleum products by the "Copper Strip Tarnish Test". This standard test was issued under the fixed designation D-130-80. The number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. This is also a standard test of the Institute of Petroleum issued under their fixed designation IP 154. This method has also been adopted for use by government agencies to replace method 5325 of Federal Test Methods Standard No. 791B.
This method covers the detection of the corrosiveness to copper of various gasolines, fuel oils, cleaners and solvents, distillate fuel oils and lubricants. The- test basically consists of placing a specially prepared copper strip blank in a copper strip corrosion test bomb constructed of stainless steel and capable of withstanding test pressures and temperatures. The lubricant is maintained in a liquid bath at a predescribed temperature, generally about 100.degree. C. for lubricant oils, and held at that temperature for about 3 hours. The copper strip is immersed in the heated lubricant oil and thereafter evaluated for copper strip applications. It is desirable to achieve classification 1-A, which exhibits a slight tarnish, light orange in color, almost the same as a freshly polished strip. The copper strip classifications are generally as follows:
TABLE 1 ______________________________________ D 130 Copper Strip Classifications Classification Designation Description.sup.A ______________________________________ Freshly -- B polished strip 1 slight tarnish a. Light orange, almost the same as freshly polished strip b. Dark orange 2 moderate tarnish a. Claret red -- b. Lavender -- c. Multicolored with lavender blue or silver, or both, overlaid on claret red -- d. Silvery -- e. Brassy or gold 3 dark tarnish a. Magenta overcast on brassy strip -- b. Multicolored with red and green showing (peacock), but no gray 4 corrosion a. Transparent black, dark gray or brown with peacock green barely showing -- b. Graphite or lusterless black -- c. Glossy or jet black ______________________________________ .sup.A The ASTM Copper Strip Corrosion Standard is a colored reproduction of strips characteristic of these descriptions. .sup.B The freshly polished strip is included in the series only as an indication of the appearance of a properly polished strip before a test run; it is not possible to duplicate this appearance after a test even with a completely noncorrosive sample.
In general, lubricants are subjected to ASTM D-130, in order to test for the corrosiveness on the copper. The interpretation of the corrosiveness of the sample is according to the appearance of the test strip as it agrees with one of the standard strips of the ASTM Copper Strip Corrosion Standards. Should a strip appear to have a darker orange color than standard strip 1-B, the sample is considered as still belonging in classification 1. However, if any evidence of red color is observed, the observed strip belongs in classification 2.
In addition to ASTM D-130, another widely accepted test, the thermal oxidation stability test (TOST), test is performed on a lubricant in a heated gear box. L-60 is currently being approved as a standard lubricant testing method by ASTM. The proposed lubricant is tested within a heated gear box containing two spur gears and a test bearing while operating at a predetermined load with copper strip blanks within the gear box, maintained at about 325" F. while bubbling air at the rate of about 0.3 gallons per hour therethrough for approximately 50 hours of continuous operation. Test results are interpreted by checking weight loss of the copper strip blank, as well as checking the color changes of the front face of the catalyst strip and the gear components and bearings of the heated gear box.
While this performance test has been very helpful in the past in evaluating the appropriateness of various lubricants for transmissions, today's higher thermal operating conditions have necessitated the use of testing the synthetic lubricants which are not readily evaluated by the traditional D-130 and L-60 test as they were generated for petroleum based lubricants.
Therefore, it is an object of this invention to provide a procedure for qualifying new synthetic gear lubricants for use in transmission cases manufactured for today's trucks with extended drain intervals.